The present invention relates to a method of melting materials and an apparatus therefor, and, in particular, to a method and apparatus for melting glass, refractory oxides, silicates, or other essentially non-metalliferrous materials including the various batch materials for rock wool manufacture. Furnaces for melting such materials include the use of round, elliptical, and rectangular shaped melting shells, and graphite, molybdenum, tungsten and tin oxide melting electrodes. Lower temperature melts such as glass may also utilize combustion atmospheres above the melt surface.
These glass and non-metalliferrous oxide melting furnaces differ significantly from the metallurgical smelting furnaces in that they have essentially little if any of a molten metal layer underneath the molten slag, glass or oxide layer. Thus metallurgical furnaces are usually characterized by a relatively thick metallic layer covered by a slag layer that is kept relatively shallow by more frequent tapping than is given to the metallic layer. In these metallurgical smelting furnaces, emphasis is generally given to keeping the slag as fluid as possible in order to effect efficient separation of metallic prill from the slag and to minimize the possibility of slag foaming. Thus extensive effort is made to develop proper fluxing practice that will form slags with viscosities that can be measured in centipoises as contrasted to the non-metallurgical furnaces where viscosities of from 5 to 1000 poises are desired for subsequent melt processing steps such as fiberization or proper glass making. Furthermore, it has not been possible continuously to bottom tap such metallurgical furnaces because of the need for collecting a significant metallic pool that inter-reacts chemically with the slag layer.
As noted above, the non-metallurgical glass and oxide melting furnaces differ greatly in their exact melting equipment and practices. However such furnaces all share common disadvantages including:
long residence times of the melt in the furnace; PA1 relatively large furnace sizes; PA1 high energy losses; PA1 selective melting of components giving an unmelted scum; PA1 the inhomogeneous nature of the emergent melt; PA1 the lack of flexibility in stopping the furnace quickly and in changing of melt composition PA1 furnace hearth ratings such as the number of square feet of hearth area required to melt one ton of batch per day; PA1 residence time of the melt in the furnace; PA1 power or energy consumption per ton of batch melted; PA1 percent thermal efficiency of the furnace. PA1 2 to 7ft.sup.2 of hearth area per ton of batch melted per day. PA1 3 to 48 hours of residence time of melt in the furnace. PA1 700 to 1800 KWH (or BTU equivalent) per ton of batch melted. PA1 25 to 75% thermal efficiencies.
Parameters that measure the efficiencies of non-metallurgical furnaces include:
Modern glass and oxide melting furnaces generally have the following ranges for these parameters:
It has been found that the input power (or energy) density is a major factor affecting furnace performance; within limits, the higher power density, the more efficient in general will be the furnace. The above listed glass and oxide melting furnace performances have been attained in production sized furnaces at power (or energy) densities of from 7 to 20 KW/ft.sup.2 of furnace area combined within the metal shell of the furnace (or the outside of the brick lining for furnaces without metal shells). It has been found that for power densities significantly higher than about 20 KW/ft.sup.2, either the furnace lining rapidly erodes or the cooling of the metal sidewall is overwhelmed resulting in disastrous break outs through the sidewall. Also with power densities of from 20 to 25 KW/ft.sup.2, oxide or glass furnaces may tend to foam causing an uneven and discontinuous furnace operation.
For the more viscous melts of the non-metallurgical melting furnaces, these power density limitations appear to be associated with the manner in which heat is transferred from the energy source throughout the melt. For combustion atmospheres, the heat must penetrate through the surface layers of the melt first, before thermal convective currents can carry it throughout the melt volume. Thus, with high energy densities, it is easy to effect excess melt surface temperatures that cause a rapid destruction of the brick at the melt line. Such high energy densities must be avoided to prevent premature melter rebuilds.
In the case of melting using either graphite, metal or tin oxide electrodes, the heat generated between the electrodes is believed to be transferred throughout the melt by an electromagnetic stirring of the melt the intensity of which increases with increasing power density. In production sized glass and oxide melters, as the power density increases above about 20 KW/ft.sup.2 of hearth area, the electromagnetic convective currents apparently convey the excess heat to the brick or metal sidewalls faster than to the batch at the surface. Thus the brick or metal shell fails prematurely, or the melt may actually foam as the result of localized overheating because the generated heat could not be dissipated throughout the melt sufficiently fast.
To overcome these disadvantages, especially those of size and energy consumption, plasma melting devices have been proposed. These usually employ a rotating steel basin in which the melting is achieved by the use of a non-transferred arc plasma electrode, the feedstock being melted as a thin film supported either on a refractory lined wall (which due to the intensity of the plasma-heated plume tends to erode and contaminate the product) or on a bed of the feedstock material. U.S. Pat. No. 4545798 describes a method and apparatus for melting glass using a plasma melting device in which the glass is liquefied at a temperature below 1315.degree. C. and flows through a drain at the bottom of the melting vessel, the liquefied material being permitted to flow from the vessel before it becomes fully melted. Additional material is fed to the surface to maintain a substantially constant layer of the unmelted material, thereby maintaining the temperature of the melting vessel relatively low and eliminating the need for forced cooling of the vessel. This prior art device tends to be of limited throughput, cannot cope with segregations in the feed material and is not very efficient in its use of energy.
We have now developed a method and apparatus for melting glass, refractory oxides, silicates, ceramics, slags and rockwool batch materials, on a continuous basis which overcomes the disadvantages of the prior art.